Year : 2009 | Volume
: 20 | Issue : 3 | Page : 443--447
Comparison of 24-hour urinary protein and protein-to-creatinine ratio in the assessment of proteinuria
Ayman M Wahbeh, Mohammad H Ewais, Mahamed E Elsharif
Division of Nephrology, Department of Internal Medicine, University of Jordan, Amman, Jordan
Ayman M Wahbeh
Assistant Professor of Medicine, University of Jordan, P.O. Box 1374, Amman 11941
To determine the correlation between protein-to-creatinine ratio (PCR) and 24-hour urinary protein (UP), we measured proteinuria in 68 patients attending the nephrology clinic at Jordan University Hospital by 24-hour urine protein excretion and protein-to-creatinine ratio. The cutoff values for spot urine protein-to-creatinine ratio in predicting 24-hour protein «DQ»threshold«DQ» excretion of 0.5, 1.0 and 3.5 g/day were determined using receiver operating characteristic curves. A very good correlation (r= 0.832, P< 0.0001) was found between spot urine protein-to-creatinine ratio and 24-hour urine protein excretion. Bland-Altman plot showed the two tests had reasonable limits of agreement at low level of protein excretion but the limits became wider as the protein excretion increased. For protein excretion < 2.0 g/day, the limits of agreement of spot urine (PCR) and (UP) were +1.48 and -1.2 g/day. The spot urine protein-to-creatinine ratios of 0.72 (sensitivity 0.97; specificity 1.0), 1.2 (0.97; 0.89) and 3.23 (1.0; 0.86) mg/mg reliably predicted 24-hour urine total protein equivalent «DQ»thresholds«DQ» of 0.5, 1.0 and 3.5 g/day, respectively. We conclude that the protein-to-creatinine ratio in spot urine specimens is an accurate, convenient, and reliable method to estimate the protein excretion in urine. However, the protein-to-creatinine ratio will likely be within clinically acceptable limits only when proteinuria is at reasonably low levels.
|How to cite this article:|
Wahbeh AM, Ewais MH, Elsharif ME. Comparison of 24-hour urinary protein and protein-to-creatinine ratio in the assessment of proteinuria.Saudi J Kidney Dis Transpl 2009;20:443-447
|How to cite this URL:|
Wahbeh AM, Ewais MH, Elsharif ME. Comparison of 24-hour urinary protein and protein-to-creatinine ratio in the assessment of proteinuria. Saudi J Kidney Dis Transpl [serial online] 2009 [cited 2020 May 26 ];20:443-447
Available from: http://www.sjkdt.org/text.asp?2009/20/3/443/50776
Measurement of protein excretion in a 24-hour urinary collection (UP) is the gold standard for the quantitative evaluation of proteinuria. However, this method is cumbersome and inconvenient, since it is difficult to collect a complete 24hour urine sample accurately, especially in the outpatient setting. An alternative method for quantitative evaluation of proteinuria is the measurement of protein-to-creatinine ratio (PCR) in an untimed spot urine specimen, which provides a more convenient method to assess protein excretion and is recommended by NKF K/DOQI guidelines. 
Although there is moderate to high correlation between PCR and UP, ,,,,, the agreement between these two measuring techniques should be assessed when considering replacing one another.  Few studies have tested this agreement with variable results. ,,
We aim in the present study to correlate the PCR and UP agreement, and attempt to find a discriminant value for PCR that reliably determines a significant threshold level of proteinuria.
Subjects and Methods
Sixty-eight outpatients with proteinuria attending the nephrology outpatient clinic at Jordan University Hospital were evaluated. Although all patients were given clear instructions how to collect the urine accurately, 18 patients were excluded because the urine collection was inadequate in 16 and over collected in two. The remaining 50 patients (29 men and 21 women) were included. Their average age was 51.1 ±17.0 years. The mean protein excretion at the time of the study was 2.35 ±2.47 gm and the mean PCR was 2.64 ±2.75 mg/mg. The specimens of 24hour urine collections and random urine specimens were collected within two days period.
The concentration of total protein in urine was measured by the turbidometric assay using benzethonium chloride, and the urine creatinine was measured by a creatinine Jaffe test using Roche/Hitachi 917 analyzer.
Data analysis was performed using MedCalc statistical software version 22.214.171.124 (demo version). Spearman's correlation between the spot urine protein-to-creatinine ratio (PCR) and 24hour urine total protein (UP) was used. The limits of agreement between the two methods were analyzed by the Bland-Altman method, , and inter-rater agreement Kappa. The discriminant cutoff values, sensitivity, and specificity of PCR were tested for predicting 24-hour protein excretion "threshold" of 0.5, 1.0 and 3.5 g/day by receiver operating characteristic (ROC) curves. 
There was a very good correlation between spot urine (PCR) and 24-hour urine total protein (UP) (r= 0.832, P= 0.0001), [Figure 1]. Wide deviation from the line of identity was noticed at high protein excretion levels. As shown on Bland-Altman plot, the limits of agreement between PCR and UP were wide at high levels of protein excretion. These limits were better and similar across a wide range of protein excretion when data were log-transformed, [Figure 2]. For protein excretion ,, renal transplant, ,, and pregnancy.  The NKF K/DOQI guidelines suggests that untimed spot urine samples should be used to detect and monitor proteinuria in children and adults, it prefers a first-morning sample, but accepts a random sample if a first-morning specimen is not available.
The findings of our study showed a very good correlation between UP and PCR. To use both tests interchangeably, it is important to demonstrate that both methods agree sufficiently. Few studies have previously assessed agreements rather than correlations between these tests and found wide limits. ,, In our study, the limits of agreement were also wide, but similar across a wide range of protein excretion when data were log-transformed, the absolute difference between PCR and UP becomes very large as protein excretion increases. The inter-rater agreement Kappa was calculated and was 0.585 (KW= 0 of 0.087 and KW # 0 of 0.053) indicating moderate agreement between both methods.
Urinary protein excretion is not constant and daily excretion varies by as much as 40% besides repeated 24-hour urine protein excretion varies by at least 15%.  Rodby et al. repeated measurements on 33 patients at least three months apart and found discordant results were the PCR increased in some patients whereas the UP fell, and vice versa.  Agrawal found a day to day variability in 24-hour urinary protein excretion of 10% and in protein-to-creatinine ratio of 2%.  This variability is a likely reason for the poor agreement between the two methods of assessing proteinuria.
By using the ROC curves, the PCR of 0.72, 1.2 and 3.28 mg/mg in spot urine specimens represents the best threshold to reliably detect urine protein excretion of 0.5, 1 and 3.5 g in 24-hour collections respectively, with high sensitivity, specificity, and area under ROC curve. The sensitivity and specificity in the present study are consistent with the previously published reports. ,,,,
We conclude that the PCR in spot urine specimens is an accurate, convenient, and reliable method to estimate the protein excretion in urine. However, when the exact amount of protein excretion is required, then a PCR will likely be within clinically acceptable limits only when proteinuria is at reasonably low levels.
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